116 EXPERIMENTS, &c. 



anterior to the retina, the middle of its picture 

 upon that membrane must be chiefly composed 

 of the less refrangible rays ; and this must be 

 the reason, that, when I look attentively at a 

 bright star with my left eye, the centre of it 

 always appears of a light orange colour. As 

 the beams, however, from the luminous point, 

 which enter the eye near to its axis, suffer but 

 little refraction, the brightness of their white 

 light, will, in great measure, overpower the 

 colour given to the middle of the picture upon 

 the retina, by the less refrangible rays of those, 

 which enter the eye at a distance from its axis. 

 Were you then to intercept the former beams, 

 the effect I have mentioned of the latter, must 

 be more observable ; and hence it is, that when 

 I place a pin or needle between my eye and 

 a luminous point, the rays of which come to 

 a focus before they fall upon the retina, the 

 shadow, instead of appearing black, is always 

 of a red or deep orange colour ; which is one 

 of the phenomena respecting luminous points, 

 to which I have alluded. 



On the other hand, should the focus of the 

 mean refrangible rays of a luminous point lie 

 behind the retina, the middle of the picture 

 there will be principally formed of the more re- 

 frangible rays ; and if the beams, which enter 

 the eye near to its axis, be also in the present 



